This invention relates to an alcohol selective gas sensor capable of selectively detecting alcohol gas with high sensitivity and comprising a detecting electrode and a semiconductor detecting element in contact therewith.
From the point of view of controlling air in various environments such as room air in ordinary dwelling houses, it is conceivable that various gases are present in the room air which include methane gas (CH.sub.4) resulting from leakage of town gas (such as natural gas), carbon monoxide (CO) gas, hydrogen (H.sub.2) gas and NOx produced by combustion, and alcohol gas due to cooking or warming of liquor. It is therefore desirable to detect alcohol such as ethanol separately from other such gases to prevent explosion of methane gas, carbon monoxide gas and hydrogen gas and poisoning by carbon monoxide gas. However, alcoholic gas tends to be detected also by a sensor intended for methane or other gas, and it is important for elimination of false alarms and malfunctioning to reduce the sensitivity of the gas sensor with respect to ethanol and other miscellaneous gases. The reduction of the sensitivity of the gas sensor per se with respect to ethanol achieved to date has not been satisfactory.
According to another method of dealing with ethanol, a sensor having no ethanol selectivity and a sensor having ethanol selectivity are combined to assure sensitivity to ethanol. An example of a known alcohol selective sensor is disclosed in Japanese patent application laid open under No. 57-118151, and this sensor comprises magnesium oxide, chromium oxide or compound oxide thereof added with silicone oxide (SiO.sub.2) and/or titanium oxide (TiO.sub.2) but has not been put to practical use. The reasons for being impractical may be that this sensor is the three-component type starting from the three types of raw material coarse powder and therefore, microscopically, its composition tends to lack in uniformity, and that because of the difference in presintering and calcination temperatures resistance in the air varies greatly.
Furthermore, according to a description in the above patent application, it is desirable to operate the gas sensor by waiting for about 24 hours after one measurement till a next measurement when measuring methane, ethanol and the like in order to eliminate hysteresis of the preceding measurement. This suggests that the disclosed gas sensor is slow in responding to methane, ethanol and the like, and particularly that it takes a long time from adsorption of these gases to desorption thereof by a detecting or sensing element in the sensor.